Transformation of Propane over ZnSnPt Modified Defective HZSM-5 Zeolites: The Crucial Role of Hydroxyl Nests Concentration

Synthesis of ZSM-5 Zeolite Nanosheets with Tunable Silanol Nest Contents across an Ultra-wide pH Range and Their Catalytic Validation

Zeolite synthesis under acidic conditions has always presented a challenge. In this study, we successfully prepared series of ZSM-5 zeolite nanosheets (Z-5-SCA-X) over a broad pH range (4 to 13) without the need for additional supplements. This achievement was realized through aggregation crystallization of ZSM-5 zeolite subcrystal (Z-5-SC) with highly short-range ordering and ultrasmall size extracted from the synthetic system of ZSM-5 zeolite. Furthermore, the crystallization behavior of Z-5-SC was investigated, revealing its non-classical crystallization process under mildly alkaline and acidic conditions (pH<10), and the combination of classical and non-classical processes under strongly alkaline conditions (pH≥10). What's particularly intriguing is that, the silanol nest content in the resultant Z-5-SCA-X samples appears to be dependent on the pH values during the Z-5-SC crystallization process rather than its crystallinity. Finally, the results of the furfuryl alcohol etherification reaction demonstrate that reducing the concentration of silanol nests significantly enhances the catalytic performance of the Z-5-SCA-X zeolite. The ability to synthesize zeolite in neutral and acidic environments without the additional mineralizing agents not only broadens the current view of traditional zeolite synthesis but also provides a new approach to control the silanol nest content of zeolite catalysts.

Disinfectant-Assisted Preparation of Hierarchical ZSM-5 Zeolite with Excellent Catalytic Stabilities in Propane Aromatization

A series of quaternary ammonium or phosphonium salts were applied as zeolite growth modifiers in the synthesis of hierarchical ZSM-5 zeolite. The results showed that the use of methyltriphenylphosphonium bromide (MTBBP) could yield nano-sized hierarchical ZSM-5 zeolite with a "rice crust" morphology feature, which demonstrates a better catalytic performance than other disinfect candidates. It was confirmed that the addition of MTBBP did not cause discernable adverse effects on the microstructures or acidities of ZSM-5, but it led to the creation of abundant meso- to marco- pores as a result of aligned tiny particle aggregations. Moreover, the generation of the special morphology was believed to be a result of the coordination and competition between MTBBP and Na+ cations. The as-synthesized hierarchical zeolite was loaded with Zn and utilized in the propane aromatization reaction, which displayed a prolonged lifetime (1430 min vs. 290 min compared with conventional ZSM-5) and an enhanced total turnover number that is four folds of the traditional one, owing to the attenuated hydride transfer reaction and slow coking rate. This work provides a new method to alter the morphological properties of zeolites with low-cost disinfectants, which is of great potential for industrial applications.

Silanol defect engineering and healing in zeolites: opportunities to fine-tune their properties and performances

Zeolites have been game-changing materials in oil refining and petrochemistry over the last 60 years and have the potential to play the same role in the emerging processes of the energy and environmental transition. Although zeolites are crystalline inorganic solids, their structures are not perfect and the presence of defect sites - mainly Brønsted acid sites and silanols - influences their thermal and chemical resistance as well as their performances in key areas such as catalysis, gas and liquid separations and ion-exchange. In this paper, we review the type of defects in zeolites and the characterization techniques used for their identification and quantification with the focus on diffraction, spectroscopic and modeling approaches. More specifically, throughout the review, we will focus on silanol (Si-OH) defects located within the micropore structure and/or on the external surface of zeolites. The main approaches applied to engineer and heal defects and their consequences on the properties and applications of zeolites in catalysis and separation processes are highlighted. Finally, the challenges and opportunities of silanol defect engineering in tuning the properties of zeolites to meet the requirements for specific applications are presented.

Relationship between Acidity and Activity on Propane Conversion over Metal-Modified HZSM-5 Catalysts

A systematic study of the comparative performances of different metal-impregnated HZSM-5 catalysts (Zn, Ga, Mo, Co, and Zr) for propane conversion is presented. The physicochemical properties of catalysts were characterized by means of XRD, BET, SEM, TEM, FTIR, XPS, 27Al MAS NMR, NH3-TPD and Py-FTIR. It was found that the acidities of the catalysts were significantly influenced by loading metal. More specifically, Mo-, Co- or Zr-modified catalysts showed a large metal size and low acidic density, resulting high olefin selectivity, while Zn- or Ga-modified catalysts maintained their small metal size and acidic density, and mainly reduced B/L due to the Lewis acid sites created by Zn or Ga species, resulting in high aromatics selectivity. Experimental results also showed that there is a balance between metals size and medium and strong acidity on propane conversion. Moreover, based on the different acidity of metal-modified HZSM-5 catalysts, the mechanism of propane conversion was also discussed.

COx co-methanation over coal combustion fly ash supported Ni-Re bimetallic catalyst: Transformation from hazardous to high value-added products

The hazardous industrial waste, coal combustion fly ash (CCFA), was creatively applied as Ni-Re bimetallic catalyst support. The expected catalyst was facilely prepared by co-impregnation method and further tested for CO_x co-methanation in a continuous fixed-bed reactor. The physico-chemical properties of the catalyst were examined by a series of techniques including XRF, ICP, XRD, N₂ isothermal adsorption, H₂-TPR, SEM and TEM. The results showed that compared to non-promoted monometallic Ni catalyst, the addition of Re promoter forming Ni-Re bimetallic catalyst was able to facilitate NiO reduction and increase Ni dispersion as well as inhibit carbon deposition and Ni sintering during reaction. The performance tests revealed that Ni₁₅Re_1.0 presented superior CO_x co-methanation activity over Ni₁₅Re_0, Ni₁₅Re_0.5 and Ni₁₅Re_1.5 due to its better anti-coking and anti-sintering ability. Based on in-situ DRIFTS analysis, a possible cycle reaction mechanism of CO_x co-methanation was reasonably proposed in the end. The reaction pathway for CO and CO₂ methanation differed from each other, where CO was linearly adsorbed on Ni metals followed by stepwise hydrogenation while CO₂ was first immobilized by the surface hydroxyl group and then gradually reacted with H₂ to form CH₄.